This document provides an overview of Android internals through a series of topics: 1. It describes key Android concepts like components, intents, and the manifest file. 2. It outlines the overall Android architecture including system startup processes like the bootloader, kernel, init, zygote and system server. 3. It covers various aspects of the Android system like the Linux kernel customizations, native user-space environment, Dalvik VM, and Java Native Interface. 4. It also profiles important system-level components like the system server, activity manager, and Binder IPC mechanism.

1. Android Internals
Android Builders Summit – April 13th 2011
Karim Yaghmour
karim.yaghmour@opersys.com
@karimyaghmour

2. About ...
● Author of:
● Introduced Linux Trace Toolkit in 1999
● Originated Adeos and relayfs (kernel/relay.c)

3. 1. Android Concepts 9. System Server
2. Overall Architecture 10.Activity Manager
3. System startup 11.Binder
4. Linux Kernel 12.Stock AOSP Apps
5. Hardware Support 13.Hacking
6. Native User-Space
7. Dalvik
8. JNI

4. 1. Android Concepts
● Components
● Intents
● Component lifecycle
● Manifest file
● Processes and threads
● Remote procedure calls

5. 1.1. Components
● 1 App = N Components
● Apps can use components of other applications
● App processes are automagically started whenever any part
is needed
● Ergo: N entry points, !1, and !main()
● Components:
● Activities
● Services
● Broadcast Receivers
● Content Providers

6. 1.2. Intents
● Intent = asynchronous message w/ or w/o
designated target
● Like a polymorphic Unix signal, but w/o
required target
● Intents “payload” held in Intent Object
● Intent Filters specified in Manifest file

7. 1.3. Component lifecycle
● System automagically starts/stops/kills
processes:
● Entire system behaviour predicated on low memory
● System triggers Lifecycle callbacks when
relevant
● Ergo: Must manage Component Lifecycle
● Some Components are more complex to
manage than others

8. 1.4. Manifest file
● Informs system about app’s components
● XML format
● Always called AndroidManifest.xml
● Activity = <activity> ... static
● Service = <service> ... static
● Broadcast Receiver:
● Static = <receiver>
● Dynamic = Context.registerReceiver()
● Content Provider = <provider> ... static

9. 1.5. Processes and threads
● Processes
● Default: all callbacks to any app Component are issued to the main process thread
● <activity>—<service>—<recipient>—<provider> have process attribute to override
default
● Do NOT perform blocking/long operations in main process thread:
– Spawn threads instead
● Process termination/restart is at system’s discretion
● Therefore:
– Must manage Component Lifecycle
● Threads:
● Create using the regular Java Thread Object
● Android API provides thread helper classes:
– Looper: for running a message loop with a thread
– Handler: for processing messages
– HandlerThread: for setting up a thread with a message loop

10. 1.6. Remote procedure calls
● Apparently System V IPC is evil ...
● Android RPCs = Binder mechanism
● Binder is a low-level functionality, not used as-is
● Instead: must define interface using Interface
Definition Language (IDL)
● IDL fed to aidl Tool to generate Java interface
definitions

11. 1.7. Development tools
● SDK:
● android – manage AVDs and SDK components
● apkbuilder – creating .apk packages
● dx – converting .jar to .dex
● adb – debug bridge
● emulator – QEMU-based ARM emulator
● ...
● Eclipse w/ ADT plugin
● NDK: GNU toolchain for native binaries

12. 2.1. Overall Architecture - EL

13. 2.2. Overall Architecture - Android

14. 3. System Startup
● Bootloader
● Kernel
● Init
● Zygote
● System Server
● Activity Manager
● Launcher (Home)

15. 3.1. Bootloader
● aosp/bootable/bootloader
● Custom bootloader for Android
● USB-based
● Implements the “fastboot” protocol
● Controlled via “fastboot” cli tool on host
● aosp/bootable/recovery
● UI-based recovery boot program
● Accessed through magic key sequence at boot
● Usually manufacturer specific variant

16. ● Flash layout:
0x000003860000­0x000003900000 : "misc"
0x000003900000­0x000003e00000 : "recovery"
0x000003e00000­0x000004300000 : "boot" Kernel
0x000004300000­0x00000c300000 : "system" /system
0x00000c300000­0x0000183c0000 : "userdata" /data
0x0000183c0000­0x00001dd20000 : "cache" /cache
0x00001dd20000­0x00001df20000 : "kpanic"
0x00001df20000­0x00001df60000 : "dinfo"
0x00001df60000­0x00001dfc0000 : "setupdata"
0x00001dfc0000­0x00001e040000 : "splash1"
0x000000300000­0x000001680000 : "modem"
From Acer Liquid-E

17. 3.2. Kernel
● Early startup code is very hardware dependent
● Initializes environment for the running of C code
● Jumps to the architecture-independent
start_kernel() function.
● Initializes high-level kernel subsystems
● Mounts root filesystem
● Starts the init process

18. 3.3. Android Init
● Open, parses, and runs /init.rc:
● Create mountpoints and mount filesystems
● Set up filesystem permissions
● Set OOM adjustments properties
● Start daemons:
– adbd
– servicemanager (binder context manager)
– vold
– netd
– rild
– app_process -Xzygote (Zygote)
– mediaserver
– ...

19. 3.4. Zygote, etc.
● Init:
● app_process -Xzygote (Zygote)
● frameworks/base/cmds/app_process/app_main.cpp:
● runtime.start(“com.android.internal.os.Zygote”, ...
● frameworks/base/core/jni/AndroidRuntime.cpp:
● startVM()
● Call Zygote's main()
● frameworks/base/core/java/com/android/internal/os/Zy
goteInit.java:
● ...

20. ● preloadClasses()
● startSystemServer()
● ... magic ...
● Call SystemServer's run()
● frameworks/base/services/java/com/android/server
/SystemServer.java:
● Start all system services/managers
● Start ActivityManager:
– Send Intent.CATEGORY_HOME
– Launcher2 kicks in

21. 4. Linux Kernel

22. 4.1. Androidisms
● Wakelocks
● lowmem handler
● Binder
● ashmem – Anonymous Shared Memory
● RAM console
● Logger
● ...

23. 5. Hardware support
Bluetooth BlueZ through D-BUS IPC (to avoid GPL contamination it seems)
GPS Manufacturer-provided libgps.so
Wifi wpa_supplicant
Display Std framebuffer driver (/dev/fb0)
Keymaps and Keyboards Std input event (/dev/event0)
Lights Manufacturer-provided liblights.so
Backlight
Keyboard
Buttons
Battery
Notifications
Attention
Audio Manufacturer-provided libaudio.so (could use ALSA underneath ... at least as illustrated in their porting guide)
Camera Manufacturer-provided libcamera.so (could use V4L2 kernel driver underneath ... as illustrated in porting guide)
Power Management “Wakelocks” kernel patch
Sensors Manufacturer-provided libsensors.so
Accelerometer
Magnetic Field
Orientation
Gyroscope
Light
Pressure
Temperature
Proximity
Radio Layer Interface Manufacturer-provided libril-<companyname>-<RIL version>.so

24. 6. Native User-Space
● Mainly
● /data => User data
● /system => System components
● Also found:
● /cache
● /mnt
● /sbin
● Etc.

25. ● Libs:
Bionic, SQLite, SSL, OpenGL|ES,
Non-Posix: limited Pthreads support, no SysV IPC
● Toolbox
● Daemons:
servicemanager, vold, rild, netd, adbd, ...

26. 7. Dalvik
● Sun-Java =
Java language + JVM + JDK libs
● Android Java =
Java language + Dalvik + Apache Harmony
● Target:
● Slow CPU
● Relatively low RAM
● OS without swap space
● Battery powered
● Now has JIT

27. 7.1. Dalvik's .dex files
● JVM munches on “.class” files
● Dalvik munches on “.dex” files
● .dex file = .class files post-processed by “dx”
utility
● Uncompressed .dex = 0.5 * Uncompressed .jar

28. 8. JNI – Java Native Interface
● Call gate for other languages, such as C, C++
● Equivalent to .NET's pinvoke
● Usage: include and call native code from App
● Tools = NDK ... samples included
● Check out “JNI Programmer's Guide and
Specification” - freely available PDF

29. 9. System Server
Entropy Service Device Policy Audio Service
Power Manager Status Bar Headset Observer
Activity Manager Clipboard Service Dock Observer
Telephone Registry Input Method Service UI Mode Manager Service
Package Manager NetStat Service Backup Service
Account Manager NetworkManagement Service AppWidget Service
Content Manager Connectivity Service Recognition Service
System Content Providers Throttle Service Status Bar Icons
Battery Service Accessibility Manager DiskStats Service
Lights Service Mount Service ADB Settings Observer
Vibrator Service Notification Manager
Alarm Manager Device Storage Monitor
Init Watchdog Location Manager
Sensor Service Search Service
Window Manager DropBox Service
Bluetooth Service Wallpaper Service

30. 9.1. Some stats
● frameworks/base/services/java/com/android/ser
ver:
● 3.5 M
● ~100 files
● 85 kloc
● Activity manager:
● 920K
● 30+ files
● 20 kloc

31. 9.2. Observing with “logcat”
● Find the System Server's PID
$ adb shell ps | grep system_server
system 63 32 120160 35408 ffffffff afd0c738 S system_server
● Look for its output:
$ adb logcat | grep “63)”
...
D/PowerManagerService( 63): bootCompleted
I/TelephonyRegistry( 63): notifyServiceState: 0 home Android Android 310260 UMTS CSS not supp...
I/TelephonyRegistry( 63): notifyDataConnection: state=0 isDataConnectivityPossible=false reason=null interfaceName=null
networkType=3
I/SearchManagerService( 63): Building list of searchable activities
I/WifiService( 63): WifiService trying to setNumAllowed to 11 with persist set to true
I/ActivityManager( 63): Config changed: { scale=1.0 imsi=310/260 loc=en_US touch=3 keys=2/1/2 nav=3/1 ...
I/TelephonyRegistry( 63): notifyMessageWaitingChanged: false
I/TelephonyRegistry( 63): notifyCallForwardingChanged: false
I/TelephonyRegistry( 63): notifyDataConnection: state=1 isDataConnectivityPossible=true reason=simL...
I/TelephonyRegistry( 63): notifyDataConnection: state=2 isDataConnectivityPossible=true reason=simL...
D/Tethering( 63): MasterInitialState.processMessage what=3
I/ActivityManager( 63): Start proc android.process.media for broadcast com.android.providers.downloads/.DownloadReceiver:
pid=223 uid=10002 gids={1015, 2001, 3003}
I/RecoverySystem( 63): No recovery log file
W/WindowManager( 63): App freeze timeout expired.
...

32. 9.3. Snapshot with “dumpsys”
Currently running services:
SurfaceFlinger
accessibility
account
activity
alarm
appwidget
audio
backup
...
wifi
window
-------------------------------------------------------------------------------
DUMP OF SERVICE SurfaceFlinger:
+ Layer 0x396b90
z= 21000, pos=( 0, 0), size=( 480, 800), needsBlending=1, needsDithering=1, invalidat ...
0]
name=com.android.launcher/com.android.launcher2.Launcher
client=0x391e48, identity=6
[ head= 1, available= 2, queued= 0 ] reallocMask=00000000, inUse=-1, identity=6, status=0
format= 1, [480x800:480] [480x800:480], freezeLock=0x0, dq-q-time=53756 us
...

33. 10. ActivityManager
● Start new Activities, Services
● Fetch Content Providers
● Intent broadcasting
● OOM adj. maintenance
● Application Not Responding
● Permissions
● Task management
● Lifecycle management

34. ● Ex. starting new app from Launcher:
● onClick(Launcher)
● startActivity(Activity.java)
● <Binder>
● ActivityManagerService
● startViaZygote(Process.java)
● <Socket>
● Zygote

35. 11. Binder
● CORBA/COM-like IPC
● Data sent through “parcels” in “transactions”
● Kernel-supported mechanism
● /dev/binder
● Check /proc/binder/*
● android.* API connected to System Server
through binder.

37. 12. Stock AOSP Apps
/packages/apps /packages/providers /packages/inputmethods
AccountsAndSettings Launcher2 ApplicationProvider LatinIME
AlarmClock Mms CalendarProvider OpenWnn
Bluetooth Music ContactsProvider PinyinIME
Browser PackageInstaller DownloadProvider
Calculator Protips DrmProvider
Calendar Provision GoogleContactsProvider
Camera QuickSearchBox MediaProvider
CertInstaller Settings TelephonyProvider
Contacts SoundRecorder UserDictionaryProvider
DeskClock SpeechRecorder
Email Stk
Gallery VoiceDialer
HTMLViewer

38. 13. Hacking
● Source:
● AOSP – source.android.com / android.git.kernel.org
● Cyanogenmod – www.cyanogenmod.com
● xdadevelopers – www.xda-developers.com
● Tools:
● repo / git
● fastboot
● recovery
● Kernel privilege escalation exploits -- “one-click root”
● ...

39. Thank you ...
karim.yaghmour@opersys.com